Intra-subject variability of platelet alpha 2-adrenoceptor binding sites in healthy volunteers.

The specific alpha 2-adrenoceptor binding (Bmax and KD) of (3H-methyl)yohimbine to intact platelets of healthy young volunteers was not significantly different in 7 males and 7 females. Its extent was not dependent on the menstrual cycle. The intraindividual variability was relatively high with means between 6% and 38% for Bmax and between 10% and 39% for KD, respectively. An intra-assay variation of 5.4 to 12.4% for Bmax and of 5.1 to 19.6% for KD contributed to this variability. Therefore, drug effects on platelet alpha 2-adrenoceptor binding in intraindividual cross-over studies are evidential only if the drug-induced changes exceed the intra-subject variability. This should be examined by at least 3 estimates of pre-drug values.